A directional microphone system is a microphone system having a directivity pattern. The directivity pattern describes the directional microphone system's sensitivity to sound pressure from different directions. The purpose of the directional microphone system is to receive sound pressure originating from a desirable sound source, such as speech, and attenuate sound pressure originating from an undesirable sound source, such as noise. The directional microphone system is typically used in noisy environments, such as in a vehicle or in a public place. An advantage of the directional microphone system is that the directivity pattern of the directional microphone system can be made more specific than that achieved through the use of a discrete microphone.
The directional microphone system generally includes a plurality of discrete microphones, each characterized by a directivity pattern, and a processor to produce the directivity pattern. Each discrete microphone produces an electrical signal responsive to sound pressure originating from both the desired and undesired sound sources. The processor processes the electrical signal from each microphone to produce an output signal having the directivity pattern of the directional microphone system.
One type of directional microphone system is a gradient directional microphone system. The gradient directional microphone system is similar to directional microphone systems except that the directivity pattern of the gradient directional microphone system is responsive to the difference in sound pressure between two discrete microphones. Because the gradient directional microphone system is responsive to the difference in sound pressure between two discrete microphones, the discrete microphones are generally located on a common axis with the desired sound source. Otherwise, the sound pressure at each discrete microphone would arrive at the same time. The gradient directional microphone system is advantageously used when the space and processing complexity for a particular application limits the number of discrete microphones.
Gradient directional microphone systems are characterized by a gradient order which defines the directivity pattern of the system. The gradient order of a gradient directional microphone system defines the degree of directionality of the system. In general, the higher the gradient order of the system the more directional the gradient directional microphone system becomes. For example, a gradient directional microphone system having a gradient order of zero implies an omnidirectional system having a directivity pattern in the shape of a circle. For example, a gradient directional microphone system having a gradient order of one can generate a directivity pattern anywhere between a figure eight pattern and a cardiod pattern. For example, a gradient directional microphone system having a gradient order of two generates a directivity pattern that can be represented as the product of the directivity pattern from two first order gradients.
A problem with the gradient directional microphone system is that the size and complexity, and therefore cost, of the system increases as the gradient order of the system increases. The size increases because additional discrete microphones are needed. The complexity increases because the processor processes electrical signals from the additional discrete microphones. The problem typically occurs when the gradient directional microphone system has a gradient order of two or more.
In the prior art, gradient directional microphone systems having a second order gradient comprise no less than four microphone ports. In one embodiment, the four microphone ports are constructed using four discrete microphones, wherein each discrete microphone has a zero order gradient. A disadvantage with using the four microphones is the space required for each discrete microphone and the distance required between adjacent discrete microphones.
In another embodiment, the four microphone ports are constructed using two discrete microphones, wherein each discrete microphone has a first order gradient and has dual microphone ports. A baffle may be placed between the dual microphone ports to separate the dual microphone ports. If a baffle is not used, the distance between the two discrete microphones must be increased beyond that need with a baffle. A disadvantage with using the four microphone ports constructed using two discrete microphones is that the baffle consumes space or that the distance between the discrete microphones is increased.
In both prior art embodiments, the processor requires the complexity necessary to process signals received from four microphone ports.
Accordingly, there is a need for a gradient directional microphone system having smaller size and less complexity.